The invention relates to a method of growth of CdTe on silicon by molecular beam epitaxy.
Epitaxial growth of CdTe on a silicon substrate has a great deal of attention as high quality and large size silicon substrates may commercially be available with a relatively low cost. Si(001) substrate or Si(001)off substrate is available for CdTe growth by use of molecular beam epitaxy, which is disclosed in J. Vacuum Sci. Technol., B10 (1992) pp. 1450, "Current status of direct growth of CdTe and HgCdTe on silicon by molecular beam epitaxy" or Society of photo-optical instrumentation engineers Vol. 1512 pp. 155 1991. According to this prior arts, a thin protective oxide film is previously formed on a surface of the silicon substrate for subsequent introduction thereof into a growth chamber for epitaxial growth of CdTe. The thin protective oxide film is removed by a heat treatment at 850.degree. C. and then the surface of the silicon substrate from which the protection oxide film was removed is subjected to a rinse. The silicon substrate is cooled down to a growth temperature of CdTe. An epitaxial growth of CdTe on silicon substrate may be carried out at a growth temperature of approximately 300.degree. C. Alternatively, it may be available growth method that an initial growth of CdTe is carried out at a low temperature of approximately 220.degree. C. for annealing at a temperature of approximately 360.degree. C. and subsequent normal growth at a temperature of approximately 300.degree. C.
A crystal quality of CdTe epitaxially grown on a Si(001) substrate depends upon a quality of the rinsed surface of the silicon substrate. As described above, the surface of the silicon substrate is, however, rinsed at the high temperature of approximately 850.degree. C. Such a high temperature causes a degasification to be generated from a circumference of the silicon substrate. This leads to a difficulty to keep a high quality of the rinsed surface of the silicon substrate. This problem is very serious particularly when the rinse is carried out within the growth chamber for CdTe epitaxial growth since the silicon substrate is polluted with Te. In such a case, it is no longer possible to obtain a high quality crystal structure or crystal perfection.
Although it was tried to bake the growth chamber in a night for subsequent rinse treatment of the silicon substrate to combat this problem, a result thereof was in a difficulty to obtain a high quality crystal structure nor crystal perfection. Establishment of an executive cleaning room has also resulted in a difficulty to obtain a high quality crystal structure nor crystal perfection. The quality of the crystal structure may be evaluated from measurement of the full width at a half maximum (FWHM) of X-ray. In the above cases, the full width at the half maximum of X-ray is varied in the range of from several hundreds seconds to several thousands seconds. Such variations of the measured values of the full width at the half maximum of X-ray means the crystal imperfection and inferior quality of CdTe grown on the silicon substrate by molecular beam epitaxy.
The Si(001) substrate permits the CdTe(111) including a great deal of twin or twin plane to be formed thereon by use of the molecular beam epitaxy. In fact, it seems difficult to suppress any generation of the twin in grown of the CdTe(111) on the Si(001). Further, the CdTe epitaxial layer necessarily has a double domain structure in which one domain differs from the another by 90.degree.. To obtain a high quality crystal structure or a crystal perfection of the CdTe grown on the silicon substrate, it would absolutely be required to keep the CdTe from including any twin or double domain crystal structure. Namely, to comply with this requirement, it would necessarily be required to obtain a twin free and single domain CdTe layer grown on the silicon substrate. In the prior art, although it was tried to use a Si(001)off substrate, this results in an inferior quality of the crystal structure of CdTe epitaxially grown on the silicon substrate. Such inferior crystal quality of the CdTe is caused by a large stress appearing at a step due to a large lattice mismatch of 30% between the CdTe(111) and the Si(001). Whereas use of the Si(001) or the Si(001)off may provide a reduction from 19.3% down to 3.4% in the effective lattice mismatch in &lt;211&gt; direction, the reduced 3.4% lattice mismatch is larger than a 0.7% lattice mismatch of CdTe(111) on GaAs(001). The reduced 3.4% lattice mismatched CdTe on the Si(001)off is, however, regarded as having a sufficient large lattice mismatch to permit a great deal of misfit dislocations to be generated on an interface between the CdTe layer and the Si substrate.
Another conventional method of the epitaxial growth of CdTe on Si substrate is disclosed in Journal of electronic materials vol. 22, pp. 951, 1993. According to this prior art, after the thin protective oxide film as formed on the silicon substrate was removed by heating up the silicon substrate, an epitaxial growth of CdTe(111)B on Si(001)off is carried out thereby resulting in 140 seconds of the full width at the half maximum of X-ray. This growth method is yet engaged with a disadvantage as to the difficulty in obtaining a high quality of a rinsed surface of the silicon substrate. As described above, a crystal quality of CdTe epitaxially grown on a Si(001) substrate depends upon a quality of the rinsed surface of the silicon substrate. To remove the protective oxide film on the silicon substrate, it is necessary to heat up the silicon substrate over 850.degree. C. Such a high temperature causes rising a partial pressure of Te in a growth chamber for epitaxial growth of CdTe on Si substrate. The rinsed surface of the silicon substrate from which the oxide film was removed is necessarily subjected to a pollution with Te as a degasification. A CdTe growth on the Si(001) once polluted with Te results in that it is no longer possible to obtain an epitaxial growth of single crystal CdTe layers on the silicon substrate, namely an undesirable growth of CdTe(111)A on the silicon substrate. The (111)A orientation is regarded as unsuitable for CdTe growth, leading to a difficulty to obtain a single crystal structure thereof with a crystal perfection. This problem is the same as described above. In this case, it is no longer possible to obtain a high quality crystal structure or crystal perfection.
Further, it is disclosed in Applied Physics Letters Vol. 63, pp. 818, 1993 that a heating up of the silicon substrate is carried out at a temperature in the range of from approximately 400.degree. C. to 500.degree. C. to rinse a surface of the silicon substrate for subsequent epitaxial growth of CdZnTe(001)off over Si(001)off through a ZnTe buffer layer. The heating up temperature in the range of from 400.degree. C. to 500.degree. C. carried out in this prior art method is still higher than a growth temperature of CdTe in the vicinity of 300.degree. C., while 158 seconds in the full width at the half maximum was obtained. Under this growth temperature, the problem about the pollution with Te as described above has not yet been settled. Even if the heating up temperature for cleaning is reduced down under the growth temperature for CdTe, hydrogen tends to remain on the surface of the silicon substrate. The remaining hydrogen on the silicon substrate may prevent a crystal growth of CdTe thereby resulting in an inferior crystal quality of CdTe.
Still another conventional method for epitaxial growth of CdTe on the silicon substrate is disclosed in Applied Physics 18a-SL-9, 1992. This conventional CdTe growth method utilizes a Te irradiation on the surface of the substrate at a temperature in the range of from 550.degree. C. to 675.degree. C. to cover the surface of the silicon with Te for subsequent CdTe epitaxial growth on the silicon substrate. This results in a growth of a CdTe with B face on Si(001). Although the previous irradiation of Te on the Si(001) permits a growth of B-face CdTe, any previous irradiation of Te on Si(221) or Si(221)off results in no crystal growth of B-face CdTe.
Yet another conventional CdTe growth method for epitaxial growth of CdTe on the silicon substrate is disclosed in the Japanese Laid-open Patent Application No. 2-150018. According to this method, a CdTe epitaxially grown on the silicon substrate is subjected to Te atoms dissociation so as to leave the Cd layer only on the silicon substrate for subsequent re-growth of CdTe on the remaining Cd layer on the silicon substrate. This method may provide a less twin CdTe epitaxially grown on the silicon substrate.
The above CdTe growth method using the Te dissociation for subsequent re-growth of CdTc is, however, engaged with the problem as to a difficulty in adhesion of Cd atoms on the silicon substrate under the CdTe growth temperature of approximately 300.degree. C. Even at approximately 100.degree. C. of the substrate, it is difficult to leave Cd atoms only on the silicon substrate. Although it is necessary to leave Te atoms or an impurity of heavy metal as a nuclear to permit the Cd atoms to remain on the silicon substrate, the Te atoms and the impurity of heavy metals permit a growth of CdTe with A-face namely prevents a growth of CdTe with B-face necessary to obtain a high quality crystal structure of CdTe. For obtaining a high quality crystal CdTe with B-face, it would be required to prepare a clean surface of the silicon substrate free from any other atoms such as Te and Cd.